1. Field of the Invention
The present invention relates to radar devices that are mounted on vehicles such as motor and flying vehicles, and used for preventing those vehicles from colliding, for performing follow-up driving while maintaining a certain distance and so forth, and, in particular, to a radar device (FMCW radar: frequency-modulated continuous wave radar) that detects, by transmission/reception of a radar wave, a range and range-change rate (relative velocity) to targets each existing outside the vehicle, and the device's processing method.
2. Description of the Related Art
In a conventional radar device, as shown in FIG. 1, there transmitted as a radar wave is a transmission signal of a carrier-wave being frequency-modulated by a triangular modulation signal to demonstrate a periodically constant and repeated increase/decrease in signal frequency. The radar wave reflected by a target is received by the device, and at the same time, a beat signal is generated by mixing the received signal with the transmission signal. The frequency of the beat signal (beat frequency) is measured in sweep intervals each, i.e., at the time of up-chirping when the frequency of the transmission signal increases and, at the time of down-chirping when the frequency of the transmission signal decreases; and then, based on an up-chirping beat frequency fbu and a down-chirping beat frequency fbd having been measured, a range R and range-change rate V to a target is calculated by using Equations (1) and (2), respectively.
                    R        =                              cT                          4              ⁢                                                          ⁢              B                                ⁢                      (                                          f                bu                            +                              f                bd                                      )                                              (        1        )                                V        =                              c                          4              ⁢                                                          ⁢                              f                0                                              ⁢                      (                                          f                bu                            -                              f                bd                                      )                                              (        2        )            
Here, the parameter B denotes a frequency modulation width corresponding to a transmission signal; the parameter f0, the center frequency of the transmission signal; the parameter T, one periodic sweep time-interval for each of an up-chirp and a down-chirp; and the parameter c, the speed of light.
As described above, in such a conventional radar device, it is possible to detect a range and range-change rate to the target by correlating an up-chirping beat frequency fbu with a down-chirping beat frequency fbd. However, even if each of the beat frequencies obtained in up-chirping and down-chirping is a beat frequency associated with the same or identical target, a frequency offset is caused therebetween. In addition, in such a state in which a plurality of targets exists, a plurality of reception signals is obtained from the targets, and therefore a plurality of beat frequencies is generated, so that it is necessary to determine which of beat frequencies at the time of up-chirping correlates with which of beat frequencies at the time of down-chirping. As a measure to this, a method of handling an environment with a plurality of targets is disclosed in which, in order to correlate beat frequencies obtained at the time of up-chirping and at that of down-chirping, beat frequencies obtained in every up-chirping sweep period are arranged in ascending order and beat frequencies obtained in every down-chirping sweep period are arranged in ascending order, and correlating the beat frequencies obtained at the time of up-chirping and at that of down-chirping is carried out so as to maintain both of the arrangements (for example, refer to “Millimeter-wave Radar Distance and Velocity Measurement Device,” Japanese Patent Publication No. 2778864).